This invention relates to an eyeglass frame, and more particularly, an eyeglass frame in which a pad arm produced from a superelastic alloy such as Ni--Ti superelastic alloy, is adjustable to conform to the user's face. A method is disclosed for producing a Ni--Ti superelastic alloy and eyeglass frame.
In general, articles produced from a superelastic alloy such as Ni--Ti superelastic alloy, have the benefit of superelastic properties when stress applied to the articles produces strain. Because of stress-induced martensitic transformation, the articles return virtually to their original shape following removal of the applied stress, even when there is a large deformation beyond the yielding point.
Stress-strain characteristics of an alloy depend upon the conditions of heat treatment following cold work. These characteristics can be represented by stress-strain charts. For example, referring to FIGS. 1a and 1b, the two charts illustrate characteristics of alloys subjected to heat treatment in the temperature ranges of 400-500.degree. C. and 200-350.degree. C., respectively.
Various uses for such superelastic alloys have been described in the prior art, as well as methods of production. Among such items are eyeglass frames, and their component elements. For example, U.S. Pat. No. 4,772,112 and U.S. Pat. 4,896,955 both disclose methods of producing a bridge, a temple, and a pad arm to support a nose pad of an eyeglass frame. The superelastic alloy in the prior art is produced from a Ni--Ti alloy by plastic working with a working rate of at least 30%. This is followed by heat treatment for more than one hour at a temperature of less than 400.degree. C. The alloy produced demonstrates superelastic characteristics over a broader temperature range that of this invention, ie. from -20 to 80.degree. C.
Each of the various parts of an eyeglass frame, particularly a pad arm, requires adjustment to properly conform to the user's face. However, the prior-art pad arm, as disclosed in the previously referenced patents, is difficult to adjust because of its broad temperature range of superelasticity. For example, a pad arm deformed at a temperature of 100.degree. C. to produce about 4% strain, returns to its original shape upon relieving the applied stress. Strain in excess of 10% is required to alter the shape as desired. This degree of strain may cause not only the pad arm to break, but also the eyeglass frame itself and the joint portion between the eyeglass frame and the pad arm.